Vol. 45 No.3 - Highlights

COMPASS: Radiative widths of the a2(1320) and π2(1670) (Vol. 45 No.3)

Intensity of the 2−+ partial-wave in π−γ ->π−π−π+, interpreted as radiative
width of the π2(1670)

Radiative transitions are among the most important and insightful processes for the investigation of atomic, nuclear and hadronic systems. They reveal the electromagnetic substructure of the involved particles. The meson is known since the 1980s to decay radiatively with a branching of about 0.3% into a pion and a photon. Theoretically this can be linked, for example through the vector meson dominance model, to the main hadronic decay channels.

Experimentally, it is difficult to observe rare decays involving a single photon directly, over an abundant neutral pion background. The way out is to measure the reverse process, the production of the resonance in a pion-photon collision.
The COMPASS experiment at CERN has taken high-statistics data of pion-nucleus collisions π-Pb->π-π-π+Pb of which the photon-exchange contribution, so-called Primakoff scattering, was singled out.. The radiative coupling of the a2(1320) resonance was determined with unprecedented precision, employing partial-wave analysis of the three-pion final state. The latter proved the radiative coupling of the 10-times less intensive π2(1670) resonance, π2(1670) -> πγ, and made possible the first measurement of its radiative width. This challenges theoretical descriptions, that aim at linking this transition intensity to the inner structure of the π2(1670).

All paths lead to Rome, even in condensed matter theory (Vol. 45 No.3)

Carlo Di Castro

The author shares his thoughts on the development of theoretical condensed matter physics in Rome from the 1960s until the beginning of this century.

This work presents his recollections of how theoretical condensed matter physics developed in Rome, starting in the 1960s. This was done in an interview in which the author reflects upon his research career.

He presents a unique, personal account of the evolution of these research fields since the 1960s. He relates the encounters he had with those who would go on to become the next generation of condensed matter physicists and explains his involvement in setting up the ‘Rome Group’, an authority in his field, together with other members.

In modern plasma applications an independent control of ion energy and ion flux is desirable to best meet process requirements. Using rf-plasma excitation at least two different frequencies need to be applied, which grants an independent modulation of ion flux and ion energy. By applying a fundamental frequency and at least one even harmonic, the relative phase shift between both frequencies directly influences the self-bias voltage. This effect is called the “Electrical Asymmetry Effect“, since the absolute values of the minimum and maximum of the resulting waveform differ from each other depending on the relative phase shift. Using the Multipole Resonance Probe, a constant electron density in the plasma bulk is determined over a wide range of applied relative phase shifts with constant voltage amplitudes of both frequencies. This directly leads to an ion flux towards the surface, which is independent from the relative phase shift, whereas the ion energy is set by the phase dependent self-bias voltage. Nevertheless, it is also shown, that this independent control is partly limited by self-excited harmonics in the process.

Low-dimensional analogue of holographic baryons (Vol. 45 No.3)

A triple chain soliton solution at high density, displaying the baryonic popcorn phenomenon in the low-dimensional theory.

Gauge/gravity duality provides a method to study various features of a large class of strongly coupled quantum field theories. A significant motivating application for these ideas is the physics of non-perturbative phases of Quantum Chromodynamics (QCD) and this is the subject of holographic QCD (HQCD). Within HQCD baryons are described by collective excitations, known as solitons, and the study of nuclei and dense QCD translates into the investigation of multi-soliton physics in a curved spacetime with an additional spatial dimension (the holographic direction). However, even computing the classical multi-soliton solutions is a difficult problem that has not yet been solved.

The authors introduce a low-dimensional analogue of holographic baryons, with the advantage that numerical computation of multi-solitons and finite density solutions is tractable. They find that many of the conjectured features of soliton physics in HQCD are realized in this model, including a series of transitions at increasing density (dubbed baryonic popcorn) where the soliton crystal develops additional layers in the holographic direction: a phenomenon that is expected to play a vital role in understanding the important issue of dense HQCD.

Plasma tool for destroying cancer cells (Vol. 45 No.3)

The plasma discharged used to study DNA at ambient air conditions. Credit: Han et al.

Inducing biological tissue damage with an atmospheric pressure plasma source could open the door to many applications in medicine
The authors conducted a quantitative and qualitative study of the different types of DNA damage induced by atmospheric pressure plasma exposure. This approach, they hope, could ultimately lead to devising alternative tools for cancer therapy as well as applications in hospital hygiene, dental care, skin diseases, antifungal care, chronic wounds and cosmetics treatments.

To investigate the DNA damage from so-called non-thermal Atmospheric Pressure Plasma Jet (APPJ), the team adopted a common technique used in biochemistry, called agarose gel electrophoresis. They studied the nature and level of DNA damage under two different conditions of the helium plasma source with different parameters of electric pulses.

The next step would involve investigating plasma made from helium mixtures with different molecular ratios of other gases to increase the level of radical species, such as reactive oxygen species and reactive nitrogen species, known to produce severe DNA damage. These could, ultimately, help to destroy cancerous tumour cells.

Football displays fractal dynamics (Vol. 45 No.3)

Physicists reveal that the real-time dynamics in a football game are subject to self-similarity characteristics in keeping with the laws of physics, regardless of players’ psychology and training.

Football fascinates millions of fans. Despite their seemingly arbitrary decisions, each player obeys certain rules, as they constantly adjust their positions in relation to their teammates, opponents, the ball and the goal. In this work the authors have now analysed the time-dependent fluctuation of both the ball and all players’ positions throughout an entire match.

The authors considered two previous football matches. Thanks to their analysis of the time-series variation in the ball versus the front-line movements of the players, they were the first to discover that these dynamics have a fractal nature. This finding implies that the movement of the ball/front-line at any given time has a strong influence on subsequent actions. This is due to the so-called memory effect, linked to the game’s fractal nature.

Échelon cracks in soft solids (Vol. 45 No.3)

Stepped crack surface developing from a straight notch (dashed line).

While under pure tension loading, crack surfaces are usually planar, whereas under superimposed shear they generally exhibit steps. Explaining the emergence of this ubiquitous instability remains a challenge in fracture mechanics. We study it here for a highly deformable solid (a hydrogel) and show that:
- échelon steps appear beyond a finite shear/tension threshold;
- contrary to linear elastic fracture mechanics predictions, they do not emerge homogeneously along the crack front via a direct bifurcation, but nucleate on local toughness/stiffness fluctuations. As such, the échelon instability continues the cross
-hatching one, observed on soft solids under pure tension, here biased by shear loading.

We argue that this behavior results from the controlling role of elastic non-linearity.

These results point to the importance of studying whether they remain relevant for stiffer materials, in order to assess the validity limit of the linear elastic approximation.

In real optical waveguides, fabrication tolerances cause the unavoidable appearance of sidewall roughness, that is a local and random deviation of the waveguide width from its nominal value. The interaction of the light propagating in the waveguide with sidewall roughness induces a coupling mechanism which transfers part of the optical power from the propagating mode(s) to other guided modes (propagating and counter-propagating) and radiative modes. Backscattered and radiated light result in what is usually referred to as extrinsic loss, which is typically the dominant loss contribution of optical waveguides.

In this paper, we formulate a novel unified vision for these roughness-related impairments (referred to as the nw model), revealing for the first time that, given the roughness properties at the waveguide interface, both backscattering and radiative losses depend only on the sensitivity of the mode effective index to waveguide width perturbation. This result finds general application to both 2D and 3D waveguide structures and is not related to any particular technology or waveguide shape. Further, it provides a key instrument for a deeper understanding of roughness induced scattering as well as simple design rules for the mitigation of waveguide extrinsic loss.

Scaling up renewable energy (Vol. 45 No.3)

Power increments of the grid measured every 15 minutes, against the initial grid power for solar power

A new study focuses on the feasibility of scaling up renewable energy to cover the needs of a country the size of Germany.

Can renewable energy adequately supply the power grid, despite its intermittent nature? This is the key question in a new study presented in this work. It outlines the key issues associated with the use of renewable energy on a large scale.

The author scaled the 2012 German national grid data—including wind (8%) and solar sources (4.8%) contributions—in such a way that renewable energy constitutes a larger than actual share of electricity production, reaching up to 100%, thus covering the country’s yearly electricity needs. The power infrastructure would have to deliver three times the energy load at peak use.

This leads to excess power production, sometimes incurring negative demand-led prices when supply significantly exceeds demand. This setup still requires backup power from thermal power plants to cover periods of low wind and solar energy production.

Making the most of carbon nanotube-liquid crystal combos (Vol. 45 No.3)

This work focuses on the influence of temperature and nanotube concentration on the physical properties of such combined materials. These findings could have implications for optimizing these combinations for non-display applications, such as sensors or externally stimulated switches, and novel materials that are responsive to electric, magnetic, mechanical or even optical fields.

In this study, the authors focused on the electro-optic and dielectric properties of ferroelectric liquid crystal-multiwall carbon nanotube combinations. Specifically, they studied the influence of temperature on the compound material’s main physical properties. They found that all dispersions exhibit the expected temperature dependencies with regard to their physical properties.

They also investigated the dependence of physical characteristics on nanotube concentration, which is still the subject of several contradicting reports. For increasing nanotube concentration, they observed a decrease in tilt angle, but an increase in spontaneous polarisation.

Spin flip in ionization of highly charged ions (Vol. 45 No.3)

The qualitatively different behavior of spin in strong laser fields, within SFA (left) and the proposed theory (right). Spin effects in the tunneling regime of ionization are built up in three steps: spin precession in the bound state, during tunneling, and during the motion in continuum. Only the last two steps are included in SFA. The red, blue and green arrows indicate the initial spin, the spin after the tunneling, and the final spin, respectively. The spin quantization axis is along the laser propagation direction.

How does the electron’s spin evolve during atomic ionization in a strong laser field? A new theoretical result obtained by the authors shed light on this relativistic quantum phenomenon. It was shown that even if an electron is very tightly bound by the strong Coulomb field in a highly charged ion, the spin dynamics may still be crucially affected by a strong laser field of relatively moderate intensity, see figure. This effect is beyond the commonly accepted Strong-Field-Approximation (SFA) and can be confirmed in a challenging experiment employing collisions of highly charged ions with ultra-strong laser beams.

Spin waves in nanowires with step-modulated thickness (Vol. 45 No.3)

(a) Schematic drawing of the nanowire array and (b) comparison between the measured (points) and calculated (curves) collective SW frequency.

It is experimentally demonstrated that collective Bloch spin waves (SWs) propagate in a magnonic crystal consisting of a dense periodic array of nanowires with step-modulated thickness. The SW dispersion (frequency vs wave vector k) was measured using the Brillouin light scattering technique by sweeping the wave vector perpendicularly to the wire length. The investigated permalloy NWs have the total width of w=300 nm and periodicity a=415 nm. These nanowires consist of two sub-wires of widths w1=120 nm and w2=180 nm and thicknesses t1=25 nm and t2=50nm, respectively. Remarkably, the lowest frequency mode has an oscillating dispersion as a function of k while modes at higher frequencies have almost constant frequency values. These results have been successfully reproduced in a numerical simulation employing two-dimensional Green’s function description of the dynamic dipole field of the precessing magnetization. The theory also allowed visualizing the non-trivial distribution of dynamic magnetization across the wire cross-section and estimating the Brillouin light scattering cross-section. This work can stimulate the design, tailoring, and characterization of SWs band structure in three dimensional magnonic crystals.

Topological quantum phase transitions (Vol. 45 No.3)

Transformation of edge states upon increasing the exchange field. For (b), it is a new type of topological insulator, where the QSH and QAH effect appear simultaneously.

The study of novel topological phases is the focus of intensive research efforts. Some theoretical works have recently been devoted to the understanding of the effect of staggered magnetic fluxes (SMFs) on the topological quantum phase transitions (TQPTs).

In the paper we investigate topological phases and corresponding TQPTs by introducing SMFs into the quantum spin Hall (QSH) systems. By varying the flux parameters, we find a rich variety of TQPTs between topological phases with a different number of edge states. Interestingly, some topological phases with high Chern numbers or spin Chern numbers may also appear with spin-orbit couplings.

We consider in particular the effect of exchange field and its role in driving TQPTs. It is shown that the system becomes a new type of topological insulator in a certain parameter region, where the QSH and quantum anomalous Hall (QAH) phases coexist. It is hoped that this work will deepen the understanding of topological phases and motivate further developments in this exciting and rapidly developing field.

Toward ultra-cold ion-atom chemistry (Vol. 45 No.3)

The development of cold hybrid ion-atom traps has enabled researchers to explore a new frontier; atom-ion interactions at temperatures below 1 K. In a recent theoretical study we explored the reaction pathways of cold Ca atoms in collisions with the various isotopes of the Yb ion. At cold temperatures we found that the dominant reaction mode involves the creation of the YbCa+ molecular ion with the emission of a photon. That rate was found to be largely independent of the isotope and is consistent with Langevin behaviour which predicts a constant rate with very weak isotopic dependence. In ion-atom processes at cold temperatures Langevin behaviour is generic and has been verified in laboratory studies.

In our investigation we proceeded to the ultra-cold limit and found a temperature transition region in which Langevin behaviour breaks down, and the reaction rates exhibit enhanced isotopic sensitivity. Our studies suggest that dramatic isotopic dependence in hetero-nuclear ion atom reactions will manifest as laboratory advances coax charged gases to ever colder temperature.